1. Field of the Invention
The present invention relates to an optical pickup apparatus.
2. Description of the Related Art
As an optical pickup apparatus compatible with a plurality of types of optical discs different in track pitch, there is known an optical pickup apparatus employing tracking control by an inline differential push-pull method. By the inline push-pull method, laser light emitted from a laser light source is diffracted by a diffraction grating in which regions having periodic structures different in phase from each other are joined, to generate 0th order light and ±1st order diffracted lights. The 0th order light and ±1st order diffracted lights are applied to a recording layer of the optical disc, and reflected lights thereof is received, to generate a main push-pull signal and a sub push-pull signal. Then, by generating a differential push-pull signal to obtain a tracking error signal, from the main push-pull signal and the sub push-pull signal, an offset component accompanying displacement of an objective lens or inclination of the optical disc can be effectively reduced. It is known that, in the diffraction grating, by providing a central region having a periodic structure further different in phase between the regions including the periodic structures different in phase from each other, a visual-field characteristic can be improved which indicates a deterioration rate of the differential push-pull signal when the objective lens is displaced in a tracking direction (radial direction of the optical disc) (Japanese Patent Laid-Open Publication No. 2004-145915, for example.) Also, there is known an optical pickup apparatus including a laser light source for emitting laser light having a wavelength corresponding to each of the optical discs in order to be compatible with both CD (Compact Disc) and DVD (Digital Versatile Disc) (Japanese Patent Laid-Open Publication No. 2007-220175, for example.)
For example, in order to be compatible with both a CD and DVD, in a case where a diffraction grating having a plurality of periodic structures different in phase from each other is used with a laser light source for emitting laser lights having two wavelengths, the location of the diffraction grating is, in many cases, fixed such that light emitting points are in a predetermined positional relationship. For example, as shown in FIG. 14, in an optical system including a diffraction grating 210 having a plurality of periodic structures, a collimating lens 212, and an objective lens 214, the diffraction grating is fixed at a position at which the light emitting point of laser light for DVD coincides with an optical axis of the objective lens 214. In DVDs, particularly a DVD-RAM (Digital Versatile Disc Random Access Memory) has a tendency that the visual-field characteristic deteriorates. Thus, the light emitting point of the laser light for DVD is made coincide with the optical axis of the objective lens 214, and therefore, the visual-field characteristic is restrained from deteriorating.
On the other hand, since the light emitting point of the laser light for DVD is at such a position as to coincide with the optical axis of the objective lens 214, the light emitting point of the laser light for CD is at such a position deviating from the optical axis of the objective lens 214. Thus, the visual-field characteristic deteriorates in a case of CD, and the visual-field characteristic might not satisfy a required level depending on a shift amount of the objective lens 214.